Methods of activating chromium catalysts

ABSTRACT

New methods for activating chromium catalysts for polymerization processes decrease the amount of time required for activation and increase catalyst activity. Rapid heating to a first temperature is followed by a first hold period before heating to a higher second temperature and maintaining the second temperature for a second hold period. In one aspect, the overall activation process takes less than 10 hours.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/570,521 “New Resins and Catalysts from Improved ActivationMethods,” filed May 12, 2004 and incorporated herein in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to chromium catalysts, methods foractivating chromium catalysts, and polymerization processes utilizingthese activated chromium catalysts. Chromium catalysts are usedworldwide to produce high density polyethylene.

BACKGROUND OF THE INVENTION

Chromium catalysts are used throughout the world for the polymerizationof polyethylene. Catalyst manufacturers prepare the catalysts, often byplacing the chromium on a solid support, such as alumina or silica. Thesupport helps to stabilize the activity of the chromium and allows thecatalyst to be shipped in an inactive form to the purchaser. Once thecatalyst arrives at a polymer manufacturing site, it must be activatedfor use in the polymerization process. Typical commercial activationprocesses consist of activating chromium catalysts by calcining orheating large quantities of the catalyst in dry air. Activation isperformed in some type of activation apparatus or vessel such as afluidized bed activator. This procedure may involve large and expensiveequipment in which the catalyst is heated over a period of time or“ramped up” to an activation temperature of 600-900° C. The ramp up isconducted slowly over a period of many hours and then the temperature ismaintained typically for another 12 hours. The catalyst is then cooleddown and discharged from the activator equipment. The entire proceduregenerally requires 36 hours to complete one cycle. Decreasing the amountof time required to activate the catalyst would shorten the processcycle time and increase the productivity of the catalyst activatingequipment.

SUMMARY OF THE INVENTION

The present invention relates to rapid activation methods for chromiumcatalysts and the reduction of the activation process time resulting inthe improvement of activator equipment output.

Novel activation processes of the present invention comprise subjectinga catalyst to a first stage comprising a first ramp up time to a firstdesired temperature maintaining the first temperature for a first holdperiod, followed by subjecting the catalyst to at least a second stagecomprising a second ramp up time to a second desired temperature,maintaining the second temperature for a second hold period, wherein thesecond temperature is greater than or equal to the first temperature anda total activation cycle comprises less than 30 hours.

The process can further comprise independently conducting any stage ofthe process in at least one atmosphere. The atmosphere can beindependently introduced during the ramp up time or the hold period ofany stage.

The process further comprises following the second stage with a thirdstage comprising a third ramp up time to a third temperature of lessthan 1000° C. and holding for a third hold period in an oxidizingatmosphere.

Additional aspects include catalyst activation processes comprisinginstantaneously subjecting a catalyst to a first stage comprising afirst temperature in at least one first atmosphere, maintaining thefirst temperature for a first hold period followed by instantaneouslysubjecting the catalyst to at least a second stage comprising a secondatmosphere and a second desired temperature that is greater than thefirst temperature and less than or equal to about 1000° C., andmaintaining the second temperature for a second hold period.

Another aspect of the invention includes chromium catalyst activationprocesses comprising at least 2 stages wherein a first stage comprisesinstantaneously introducing a catalyst to an ambient atmospherepreviously heated to a temperature of about 600° C., holding thetemperature for about 1 hour to about 20 hours, followed by a secondstage comprising raising the temperature to a range from about 800° C.to about 900° C. over a period of about 0 hours to about 3 hours,holding the temperature for a range of about 1 hour to about 10 hours,wherein the process is performed in a batch mode or a continuous mode.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graphic representation of the sensitivity of achromium/silica catalyst to moisture at various temperatures.

FIG. 2 is a graphic representation of thermogravimetric analysisdemonstrating the equivalence of time and temperature in the dehydrationof silica.

FIG. 3 is a graphic representation of thermogravimetric analysisdemonstrating the weight loss of catalyst at various temperatures duringthe isothermal hold period.

FIG. 4 is a graphic representation demonstrating the maximum weight lossof catalyst during the isothermal hold.

DETAILED DESCRIPTION OF THE INVENTION

The purpose of activation of chromium catalysts is dehydroxylation ofthe catalyst support and oxidation of any of the trivalent form ofchromium, Cr(+3) to the hexavalent form, Cr (+6) and then stabilizationof the Cr (+6) form. For purposes of the invention, the term“stabilization” refers to this activation process resulting in theChromium(+6) form of the catalyst. Prior to activation, a commercialcatalyst can contain a total of about 0.2 to about 2.0% trivalentchromium by weight. Most frequently such catalysts contain about 1% Crby weight. From these 1% Cr catalysts, a stabilization process yielding0.6 to 0.8 wt % Cr(6+) is considered desirable. That is, it isconsidered desirable when at least 60% of the total Cr is converted toCr(+6) during the activation. This value is referred to herein as thepercent conversion to Cr(+6). For efficiency, the present commercialprocess activates catalysts in large volumes, for example, 500 to 1200pounds (363 to 544 Kg), and requires slowly heating up to a hightemperature, typically around 800° C., over a long period of time,typically around 36 hours. These high volume commercial activations ataround 800° C. usually produce catalysts containing from about 0.4 toabout 0.6% Cr(+6) and can require 36 hours to complete. In general, anactivation process that yields greater than 0.4 wt. % (or 40%conversion) Cr(+6) is considered commercially acceptable.

The activation cycle is extensive because Cr (+6) is thermally unstable.The hexavalent oxide itself, CrO₃, decomposes into Cr(3+) and O₂ whenheated above 200° C. When chromium is placed on a support such as asilica or alumina surface, the compound is stabilized and can endureeven up to 900° C. if there is an absence of moisture. Moisture is knownto be a significant impediment to achieving high stabilization toCr(+6). When even traces of moisture are present, the chromate ester canbe hydrolyzed and Cr (+6) can again decompose to Cr (3+). Placing thechromium on a support does not eliminate the influence of the moisturebecause the support surface, for example, silica, releases moisture asit is heated to progressively higher temperatures. The deeper the areacontaining the catalyst in the catalyst activator equipment (referred toas the catalyst bed), the more moisture is released. This makes itdifficult to achieve high stabilization and high output simultaneouslyin a commercial activator. Therefore, commercial activation methodsgenerally proceed over extended periods of time to minimize the effectof moisture on the catalyst and the Cr(+6) stabilization. The totalactivation of the catalyst at high temperature typically takes a totalof 36 hours.

The present invention relates to rapid activation methods for chromiumcatalysts, resulting in the reduction of the activator process time andthe improvement of activator equipment output.

Various types of activator equipment or apparatus can be used toactivate catalysts for the present invention. Such equipment can includeany vessel or apparatus including, but not limited to, rotary calciners,static pan drying, and fluidized beds. Such equipment can operate in astatic or batch mode, or in a continuous mode. In the case of a staticor batch mode, a vessel or apparatus containing the catalyst bed can besubjected sequentially to various stages of the activation process. Fora continuous mode, the stages of the process can occur in a series ofzones through which the catalyst passes on its path through theactivation apparatus.

In a fluidized bed activator, gas flows upward through a grid platecontaining many small holes upon which the supported catalyst ispositioned. The gas can contain various compounds to create a desirableatmosphere. The catalyst is mixed in the gas as it flows creating afluid-like flow. This is often referred to as fluidization orfluidizing. The gas flow for fluidized bed activators can range fromabout 0.01 to about 1 foot per second (0.01 to 30 cm/sec).Alternatively, the gas velocity can range from about 0.05 to about 0.5ft/sec (1.5 to 15 cm.sec), or from about 0.1 to about 0.3 ft/sec (3-9cm/sec).

Catalysts suitable for the present invention include any catalystssuitable for polymerizing polyolefins and comprising chromium on asupport. The chromium content can range from about 0.1 to about 10% byweight based upon the total weight of the catalyst. Alternatively, thechromium content can range from about 0.2 to about 5% by weight, orabout 0.5 to about 2% by weight. Suitable supports for chromiumcatalysts of the present invention include, but are not limited to,silica, alumina, aluminophosphates, metal oxides such as oxides oftitanium, zirconium, boron, zinc, magnesium, and the like, orcombinations thereof. Suitable supports may also contain other promotersincluding, but not limited to, fluoride, sulfate, fluoroborates,silicofluorides and the like. Suitable catalysts can be purchased fromcommercial sources such as the Grace Davison Division of W. R. Grace &Company, Columbia, Md.

Catalysts activated according to the present invention typically have 40to 100% of the total chromium converted to the hexavalent form afteractivation at 750° C. to 900° C. In another aspect, conversion of 50% to100% of total Cr to Cr(+6) is achieved. In yet another aspect,conversions of 60% to 100% are achieved at 750° C. to 900° C. and instill another aspect conversions of 80 to 100% are achieved. It is knownthat high conversions are more easily achieved at lower temperatures(see M. P. McDaniel; The State of Cr(VI) on the Phillips PolymerizationCatalyst IV. Saturation Coverage; J. Catal. 76, 37 (1982)).

Activations conducted according to the present invention compriseheating the catalyst to a final desired temperature in two or optionallythree stages. For purposes of the invention, the term “stage” refers toheating a catalyst to a desired temperature and then maintaining thattemperature for a period of time. A stage can be performed when thecatalyst is in a stationary position or by moving the catalyst throughvarious locations. A first stage comprises a first ramp up time (R1) toa first desired temperature (T1), and maintaining the catalyst at thattemperature for a first hold period (H1). For purposes of the invention,the term “ramp up time” refers to a period of time over which thetemperature is increased and the terms “hold time” and “hold period” areconsidered interchangeable. Following the first stage the catalyst issubjected to at least a second stage comprising a second ramp up time(R2), to a second temperature (T2), and a second hold period (H2). Theactivation processes of the present invention further compriseindependently conducting any stage of the process in at least oneatmosphere. The atmosphere can be independently introduced during theramp up time or the hold period of any stage.

Optionally, the process comprises a third stage comprising a third rampup time (R3), to a third temperature, (T3) and a third hold period (H3).According to this invention it can be advantageous for the third stageor the last performed stage of the process to be conducted in anoxidizing atmosphere.

Any of the aspects of the present invention can further comprise a finalreducing treatment following the last holding period of the activationprocess. The final reducing treatment comprises introducing the catalystto a reducing atmosphere for a period of time ranging from about 10minutes to about 5 hours. The reducing atmosphere can comprise pure COor CO in mixtures with other inert gasses. Introducing the catalyst tothe reducing atmosphere can comprise exchanging the atmosphere within anactivation vessel, or any other commercially acceptable process forintroducing the catalyst to the reducing atmosphere. The reducing stepcan be performed in temperatures ranging from about 200° C. to about500° C.

The ramp up times, R1, R2 and R3, can be the same amount of time or theycan each be independently different. The range for any ramp up time canbe instantaneous to less than or equal to 3 hours. In one aspect of thisinvention any ramp up time can be less than 3 hours. In another aspectany ramp up time can be less than or equal to about one hour. In anotheraspect, any ramp up time can be instantaneous, or about zero hours. Whenthe ramp up time is instantaneous or about 0 hours, the catalyst isintroduced into a pre-heated environment. For purposes of the invention,the term “instantaneous” refers to introducing the catalyst to apre-heated environment wherein the ramp up time would be negligible orabout 0 hours.

For purposes of the invention, the first temperature can be in a rangeof less than or equal to 500° C. to less than or equal to 700° C. In oneaspect of this invention the first stage temperature (T1) is less thanabout 700° C. In another aspect, T1 is less than about 600° C. and instill another aspect, T1 is less than about 500° C. The second stagetemperature (T2) can be any temperature greater than or equal to T1 andless than or equal to 1000° C. In one aspect of this invention T2 isless than about 1000° C. In another aspect T2 is from about 600° C. toabout 900° C. In still another aspect T2 is from about 700° C. to about870° C. In still another aspect T2 is 750° C. to 850° C. For any thirdstage, the third temperature (T3) is the highest temperature to whichthe catalyst is exposed and is less than or equal to 1000° C. In oneaspect of this invention T3 is less than about 1000° C. In anotheraspect T3 is from about 600° C. to about 900° C. In still another aspectT3 is from about 700° C. to about 870° C. In still another aspect T3 is750° C. to 850° C.

The hold periods can also be varied independently. That is, H1, H2 andH3 can be the same or different. Any hold period can range from 1 minuteto about 30 hours. In one aspect any hold period can range from about 1minute to about 30 hours. In another aspect any hold period can rangefrom about 10 minutes to about 8 hours, and in another aspect any holdperiod can range from about 30 minutes to about 3 hours.

Activation of catalysts of the present invention can occur in variousatmospheres. The atmosphere can vary independently during various stagesof the activation process, or the atmosphere can remain consistentthroughout all stages of the process. Atmospheres can compriseoxidizing, inert, and reducing compounds. The relevant atmosphere can beintroduced independently to the activation process during any part ofany stage, such as during the ramp up time or during the hold period forany stage of the process. For example, an atmosphere can be introducedduring the hold period of the first stage and an atmosphere can beintroduced during the ramp up time of the second stage and then anotheratmosphere introduced during the hold period of the second stage.

Each stage can be conducted independently in an oxidizing, an inert, ora reducing atmosphere, however, it is advantageous for the final stageof the process to be conducted in an oxidizing atmosphere. When theinvention comprises a third stage, it can be advantageous to use anoxidizing atmosphere.

This invention further comprises optionally following a last stage of aprocess with a step comprising subjecting the catalyst to a reduction at200° C. to 500° C. in a reducing atmosphere such as carbon monoxide forabout 10 minutes to about 5 hours.

Oxidizing atmospheres can include pure oxygen or ambient air containingoxygen. Substantially anhydrous air can be used. The term dry air refersto substantially anhydrous air. Gas containing from about 5 to about100% oxygen can be used. Alternatively the gas can contain from about 10to about 50% oxygen, or from about 15% to about 30% oxygen. For purposesof the invention, the term “air” refers to an oxidizing atmosphere.Other oxidizing compounds that can be used in an oxidizing atmosphereinclude, but are not limited to, nitrous oxide (N₂O), nitrogen dioxide(NO₂), nitric oxide (NO), oxygen containing halide compounds such asiodine pentoxide (I₂O₅) or chlorine monoxide (Cl₂O) and other materialswhich release oxygen. An oxidizing atmosphere can comprise anycombinations of the foregoing compounds.

Inert atmospheres can include, but are not limited to, carbon dioxide(CO₂), vacuum, helium, argon and nitrogen. Any combination is alsoappropriate.

Reducing atmospheres include, but are not limited to, carbon monoxide(CO), hydrogen (H2), and materials that decompose to CO, C and H2.Suitable materials include hydrocarbons, alcohols, ammonia andcarboxylic acids. Any combination is also appropriate.

Combinations of atmospheres at different stages of the activationprocess can be employed for the present invention. For example, a firststage of activation can be conducted in an inert, an oxidizing or areducing atmosphere and the second stage in an oxidizing atmosphere.Likewise, the first stage can be conducted in an inert, an oxidizing ora reducing atmosphere and the second stage in a reducing atmosphere. Anyof the processes of the present invention can further compriseexchanging the atmosphere from one atmosphere to another during thefirst ramp up time or first holding period. For example, an oxidizingatmosphere of the first holding period may be exchanged for anotheroxidizing atmosphere, a reducing atmosphere or an inert atmosphere.Likewise, any inert atmosphere can be exchanged for another inertatmosphere, a reducing atmosphere or an oxidizing atmosphere. A reducingatmosphere can be exchanged for another reducing atmosphere, anoxidizing or an inert atmosphere.

The inventive process of selective shortening of ramp up and holdperiods has little consequence on catalyst quality and significantlyshortens the overall activation time, also referred to as total cycletime. Because the ramp up times R1, R2 and R3 of this invention arerelatively short, the overall activation time is decreased. In oneaspect of this invention, overall activation time (or cycle time) isless than about 20 hours. In another aspect, the overall activation timeis less than about 15 hours. In still another aspect, the overallactivation time is less than about 10 hours and in another aspect it isless than about 6 hours. In general, the overall activation time ortotal cycle time comprises the ramp up and hold times and any additionaltime required to operate the process. For example, the total cycle timecan comprise time to cool the catalyst, time to position vessels orapparatus, and the like.

According to this invention, the percent of water in the atmosphere,including the moisture released by the catalyst, is less than about 70%during the first stage of the activation process. Alternatively thewater content is less than about 50%, or less than about 10%. During thesecond stage of the activation process the percentage of water in theatmosphere can be adjusted to be less than about 30%, less than about20% or less than about 10%. During the final hold period of the lastperformed stage of the process, the amount of water in the atmospherewas less than about 50,000 ppm, less than about 10,000 ppm or less thanabout 1000 ppm.

In the case of a batch activation mode, a vessel comprising a catalystbed containing catalyst can be subjected to these stages sequentiallyand then the vessel and catalyst can be cooled and the activatedcatalyst can be discharged. Generally, the temperatures are ramped upover a period of time as the vessel is heated. Alternatively, it ispossible to perform at least one stage of the batch process bysubjecting the vessel containing the catalyst to a preheatedenvironment, so that the ramp up time is instantaneous.

In the case of a continuous activation mode, any or all of theactivation stages of the process can occur throughout a series of heatedzones, through which the catalyst passes on its path through theactivator vessel. In the case of a continuous activator, there is aninstantaneous temperature change as the catalyst moves from one heatedzone into another and the ramp times (R1, R2, and R3) would benegligible or instantaneous or about zero hours. Advantageously, thecatalyst can be added to the activation apparatus continually andremoved continually upon completion of the process when a continuousmode is used.

During activation the catalyst bed depth can range from about 0.1 toabout 20 feet (0.03 to 6 meters). In other aspects the bed depth canrange from about 1 to about 10 feet (0.3 to 3 meters), or from about 2to about 8 feet (0.6 to 2.4 meters).

In one aspect of the invention, a two-step process for decreasingchromium catalyst activation time comprises heating a chromium catalystby fluidization in dry air in a static batch process for less than 3hours to a first temperature less than 700° C., maintaining the catalystat the first temperature for a first hold period greater than 1 minute,heating in dry air for less than 1 hour to a second temperature greaterthan the first temperature, and maintaining at the second temperaturefor a second hold period greater than 1 hour. The total cycle time isless than 10 hours.

An alternative aspect of the invention comprises a batch process inwhich the catalyst is introduced in dry fluidizing air into a pre-heatedenvironment heated to a first temperature less than 700° C., maintainingthe catalyst at the first temperature for a first hold period greaterthan 1 minute, heating for less than 3 hours to a second temperaturegreater than the first temperature, still in dry air, and maintaining atthe second temperature for a second hold period greater than 1 hour. Thetotal cycle time is less than 10 hours.

Further aspects of the invention comprise a continuous activationprocess in which a chromium catalyst is introduced into a successiveseries of two or three heating zones, each set at the same or a highertemperature, in fluidizing dry air. The first zone is set at 600° C. orless, the final zone is set at 700-900° C., and the total activationtime is less than 15 hours.

In an alternative aspect of the invention, a process for activating achromium catalyst comprises using an inert or reducing fluidization gasin one or two stages of a batch or continuous process. A chromiumcatalyst is raised to a first temperature of about 700° C. or less, overa period of less than 3 hours and held at that temperature for greaterthan 1 minute. The catalyst is then heated for less than 1 hour to asecond temperature greater than the first temperature, and maintained atthe second temperature for a second hold period greater than 1 min.Finally it may, optionally, be heated to a third temperature, greaterthan or equal to, the previous temperature, and held there for a thirdhold period of greater than 1 minute. In this aspect, the catalyst isfluidized in dry inert or reducing atmosphere, in the first and/orsecond heating zone, and the final zone is in dry oxidizing atmosphere.The total cycle time is less than 15 hours.

In one aspect of the invention, increasing the output of chromiumcatalyst in an activation process is contemplated. A method forincreasing the output of chromium catalyst activation comprisesinstantaneously subjecting a catalyst to a temperature of about 800° C.in a first atmosphere of nitrogen or carbon monoxide, maintaining thetemperature for at least about 15 minutes, exchanging the firstatmosphere for anhydrous air, and holding the catalyst at thetemperature for an additional period of time.

Activated catalysts of the present invention can be used in any type ofolefin polymerization reactor known in the art. For purposes of theinvention, the term polymerization reactor includes any polymerizationreactor known in the art that is capable of polymerizing olefin monomersto produce homopolymers or copolymers of the present invention. Suchreactors can comprise slurry reactors, gas-phase reactors, solutionreactors or any combination thereof. Gas phase reactors can comprisefluidized bed reactors or tubular reactors. Slurry reactors can comprisevertical loops or horizontal loops. Solution reactors can comprisestirred tank or autoclave reactors. Such reactors can be combined intomultiple reactor systems operated in parallel or in series. The catalystalso may be used to produce ethylene polymers in a particle form processas disclosed in U.S. Pat. Nos. 3,624,063, 5,565,175 and 6,239,235 whichare incorporated by reference herein in their entirety.

A loop reactor is commonly used for a polymerization technique commonlyreferred to as particle form, or slurry process. For this process, thetemperature is kept below the temperature at which the polymer swells orgoes into solution. The temperature in the particle form process can bewithin a range of about 150° F. to about 230° F. (about 65° C. to about110° C.), although higher or lower temperatures can sometimes be used.Polymerization methods for the slurry process can employ a loop reactoror utilize a plurality of stirred reactors either in series, parallel orcombinations wherein the reaction conditions can be different in each ofthe reactors. Such polymerization techniques are disclosed in U.S. Pat.Nos. 3,248,179, 4,424,341; 4,501,885; 4,613,484; 4,737,280; and5,597,892; which are incorporated by reference herein.

Activated catalysts of the present invention can be used forpolymerization of homopolymers or copolymers from monomers. Monomersuseful in the present invention are unsaturated hydrocarbons having from2 to 20 carbon atoms. Monomers include, but are not limited to,ethylene, propylene, 1-butene, 3-methyl-1-butene, 1-pentene,3-methyl-1-pentene, 4-methyl-1-pentene, 1-hexene, 3-ethyl-1-hexene,1-heptene, 1-octene, 1-nonene, 1-decene, and mixtures thereof.

The molecular weight of the polymer can be controlled by various meansknown in the art including but not limited to, adjusting the temperature(higher temperature giving lower molecular weight) and introducing, orvarying the amount of hydrogen, or varying the catalyst compounds.

EXAMPLES

The present invention is further illustrated by the following examples,which are not to be construed in any way as imposing limitations uponthe scope of the invention. On the contrary, it is to be clearlyunderstood that resort may be had to various other aspects, embodiments,modifications, features, and equivalents thereof which, after readingthe description herein, may suggest themselves to one of ordinary skillin the art without departing from the present invention or the scope ofthe appended claims.

To demonstrate the invention, three commercially available chromiumcatalysts were selected. Both 969MS and 969 MPI catalysts were obtainedfrom W.R. Grace Company, and EP30X was obtained from Inneos Company.These three catalyst grades are considered equivalent in that they allhave a surface area of about 300 m²/g, a pore volume of about 1.6 cc/g,an average particle size of about 90-100 microns, and contain a chromiumloading of about 1 wt %.

Example 1

A laboratory procedure was designed to reproduce the longer procedureused for commercial activations. In a commercial process, about 600-750lbs (272 to 340 Kg) of catalyst was introduced or charged to a vesselhaving a grid plate 42 inches (1.1 meters) in diameter. Dry air or otheroxidizing atmosphere was blown up through the plate to fluidize thecatalyst. Air was introduced at about 0.15 ft/sec (4.6 cm/sec), and theramp up time, or period of increasing temperature, typically took about10 hours to reach 800° C. Longer ramp up times can be used commerciallyto compensate for the increased moisture generated when proportionallydeeper beds of catalyst are used. The larger catalyst charge releasesmore moisture, which can damage the catalyst. To compensate, themoisture release that accompanies the temperature rise is diluted inmore air by slowing down the ramp up rate.

To reproduce commercial conditions for the laboratory studies, moisturewas deliberately added to the fluidizing gas and controlled by bubblingthe gas through a 25° C. water column before it was used for activation.This fluidizing gas then contained 100% humidity at room temperature. Toobtain lower levels of humidity, the gas was either bubbled through anice-water column, or the levels of humidity were further diluted withdry gas before being used for the activation procedure.

For the laboratory studies, about 10 grams of commercial catalyst wasplaced in a 1.75-inch (4.4 cm) quartz tube fitted with a sintered quartzdisk at the bottom. While the catalyst was supported on the disk, thefluidization gas (air, nitrogen, or carbon monoxide) was blown upthrough the disk at the linear rate of about 0.1 to 0.3 ft/sec (3-9cm/sec). An electric furnace was placed around the quartz tube in orderto heat the tube. A thermocouple was placed inside the tube to monitortemperature, and the signal from the thermocouple was connected to anelectronic controller that also supplied current to the heater. Thecontroller could be programmed to raise the temperature of the fluidizedbed at a fixed rate, and hold it at specified temperatures for specifiedperiods of time.

For the control samples, the catalyst was fluidized in about 0.2 ft/sec(6 cm/sec) of dry air and the temperature was raised at the rate ofabout 60° C. to about 240° C. per hour to the desired temperature,typically around 800° C. At that temperature the catalyst was allowed tofluidize for three to five hours in the dry air. Afterward the catalystwas collected and stored under dry nitrogen, where it was protected fromthe atmosphere until ready for testing. Before that time it was neverallowed to experience any exposure to the atmosphere.

In an alternative control sample, the activator tube was preset andheated to the desired temperature and the catalyst was simply sprinkledinto the tube over about 30 sec. This constitutes an “instantaneous”temperature rise, sometimes referred to as a “drop in”.

Hexavalent Cr analysis was performed by adding about 1 gram of catalystinto a IN sulfuric acid solution. A few drops of 1,10-phenanthrolineiron (II) sulfate, 0.025M in water, was used as an indicator. Whilestirring, the solution was titrated with a standardized solution offerrous ammonium sulfate in water, until the indicator changed color.This solution was calibrated by reaction with potassium dichromate. Thereaction was the reduction of Cr(+6) to Cr(+3) by oxidation of Fe(+2) toFe(+3).

Polymerization runs were made in a 2.2 liter steel reactor equipped witha marine stirrer rotating at 400 rpm. The reactor was surrounded by asteel jacket containing boiling methanol and connected to a steelcondenser. The boiling point of the methanol was controlled by varyingthe nitrogen pressure applied to the condenser and jacket. Electroniccontrol instruments permitted precise temperature control to within halfa degree centigrade.

Unless otherwise stated, a small amount (about 0.01 to about 0.10 grams)of the solid catalyst was first charged or added under an inertatmosphere of nitrogen to the dry reactor. Next, 1.2 liter of isobutaneliquid was charged and the reactor heated up to the specifiedtemperature, usually around 105° C. Finally, ethylene was added to thereactor to equal a fixed pressure, normally 550 psig (3.8 MPa), whichwas maintained during the experiment. The polymerization was allowed tocontinue for the specified time, usually around one hour, and theactivity was noted by recording the flow of ethylene into the reactor tomaintain the set pressure.

After the allotted time, the ethylene flow was stopped and the reactorslowly depressurized. The reactor was opened to recover a granularpolymer powder. In all cases the reactor was clean with no indication ofany wall scale, coating, or other forms of fouling. The polymer powderwas then removed and weighed. The activity of the catalyst wascalculated as grams of polymer produced per gram of solid catalystcharged per hour.

The ethylene monomer used for the tests was polymerization gradeethylene previously obtained from the former Union Carbide Corporation.This ethylene was then further purified through a column of ¼ inch (6millimeter) beads of Alcoa A201 alumina, activated at 250° C. innitrogen. Isobutane diluent was polymerization grade previously obtainedfrom the former Phillips Petroleum Co., Borger, Tex. It was furtherpurified by distillation and it too was then passed through a column of¼ inch (6 millimeter) beads of Alcoa A201 alumina, activated at 250° C.in nitrogen.

To determine the effect of moisture on stabilization to hexavalentchromium, quartz activator tubes containing 10 grams each of 969MScatalyst were heated to various temperatures in air to which variousamounts of moisture had been added. The data is plotted in FIG. 1. Therate of temperature rise was 1500° C. per hour, so that the longest ramptime was about thirty minutes. The catalysts were fluidized in the airof varying moisture levels for three hours at the temperature indicated,and then the catalyst was cooled down and tested for stabilization toCr(+6). At 600° C. and below, stabilization was high and almostindependent of moisture level. However, at higher temperatures, themoisture level became critical. The data demonstrates that in atwo-stage activation, the first stage can be as high as 600° C. with avery fast rate of temperature rise, thus generating high moisturelevels, without damage to the catalyst. Also, above 600° C. it can beadvantageous to maintain a low moisture level during activation.

Example 2

To study the effect of moisture, the catalyst was dropped into a hotactivator tube containing nitrogen at 30,000 ppm moisture instead of theatmosphere of air used in the previous test. After 15 minutes exposure,the atmosphere was then changed to dry air, and the catalyst was exposedfor an additional 15 minutes. Results shown in FIG. 1 demonstrate thatthe Cr(+6) stabilization for the catalyst in this test was much higherin the activated catalyst than it was in a corresponding catalysttreated at 800° C. in air when both catalysts were treated at the samemoisture level. Without intending to be limited by theory, it isbelieved that nitrogen protects the catalyst and that large crystallitesof alpha-chromia grow more easily when traces of oxygen are present withmoisture. [See Excess Oxygen of Chromia, I.; by M. P. McDaniel and R. L.Burwell, Jr.; Journal of Catalysis, Vol. 36, p. 394 (1975), and ExcessOxygen of Chromia, II. Reaction with Diphenylpicrylhydrazine; by M. P.McDaniel and R. L. Burwell, Jr.; Journal of Catalysis, Vol. 36, p. 404(1975), and also The Activation of the Phillips Polymerization Catalyst,II. Activation by Reduction/Reoxidation; by M. B. Welch and M. P.McDaniel; Journal of Catalysis, Vol. 82, p. 110 (1983).] Thesecrystallites are then very difficult to reoxidize and redisperse asCr(+6). As demonstrated in FIG. 1, for a more efficient activationprocess, nitrogen or other non-oxidizing gasses can be used to removemoisture followed by a final oxidation to improve stabilization toCr(+6).

Example 3

Thermogravimetric analysis was performed to determine the effect oftemperature on hold time. The silica from the activated catalyst wassubjected to a thermogravimetic analysis. Starting at room temperatureeach sample was heated in flowing nitrogen at 40° C. per hour up to 900°C. and the weight of the sample was monitored. Each sample lost weightas the temperature was raised, due to the loss of moisture. During eachexperiment, the temperature rise was halted at various temperatures from200° C. to 800° C. where it was held for 24 hours. Afterward thetemperature rise was continued at 40° C. per hour as before up to amaximum of 900° C. The data was plotted and the curves are shown in FIG.2. The break in the curve represents the 24 hr isothermal hold period.The silica lost the most weight during the time when the temperature washalted at 500-600° C. This suggests that molecular motion begins toaccelerate at that temperature. At still higher temperatures, themolecular motion is even faster and much of the moisture is lost beforethe isothermal hold begins. The weight loss during this isothermal holdperiod is shown in FIG. 3, which demonstrates the amount of weight lossfor each temperature, and also how fast that moisture was lost. Thus, amaximum weight loss, plotted in FIG. 4, is seen at 500-600° C. Thisindicates that time and temperature can, to some degree, be consideredas interchangeable. Short hold times at high temperature are equivalent,in terms of moisture loss, to long hold times at low temperature (at500° C. and above).

Example 4

Using the information discussed above, various methods were designed foractivating chromium catalysts using multiple steps to minimize the timecycle of an activation (thus increasing the output of an activatorapparatus) or to improve the quality of the catalyst Cr(+6)stabilization. Numerous different activation sequences were tested andare described in Table 1. These catalysts were all activated using gasvelocities of 0.12 ft/sec (3.7 cm/sec). A reading of greater than orequal to 0.4% Chromium(+6) was considered commercially acceptableactivation.

TABLE 1 First Stage Second Stage Total Raise Ramp Max Ramp Cycle TempTime % Hold Raise Time Max Hold % Time Example # Catalyst To: (min)Atmos. H2O Time Temp To: (min) Atmos. % H2O Time Color Cr(+6) (min)Control 1 969MPI 800° C. 240 Air 0.2  3 hr Yellow 0.5938 420 Control 2969MPI 800° C. 1 Air 44.8 15 min Green 0.346 16 Control 3 969MPI 788° C.720 Air <1% 12 hr Green 0.4347 2160 4-1 969MPI 600° C. 3 Air 12.7 15 min800° C. 2 air 14.1 15 min Green 0.4477 35 4-2 969MPI 600° C. 1 Air 38.115 min 800° C. 1 air 24.8 30 min Green 0.4694 47 4-3 969MPI 600° C. 2Air 19.0 24 hr 800° C. 1 air 24.8 30 min Orange 0.6641 1473 4-4 EP 30X600° C. 7 Air 5.4 15 min 800° C. 1 air 24.8 15 min Orange 0.571 38 4-5969MPI 700° C. 1 Air 41.1 15 min 800° C. 1 air 16.5 30 min Green 0.493747 4-6 969MPI 700° C. 1 Air 41.1 24 hr 800° C. 1 air 16.5 30 min Orange0.5964 1472 4-7 969MPI 600° C. 1 Air 38.1  1 min 800° C. 13 air  2.5 15min Orange 0.621 30 4-8 969MSB 800° C. 1 N2 44.8  7 min 800° C. 0 air<1% 15 min Green 0.4394 23 4-9 969MSB 800° C. 1 N2 44.8 13 min 800° C. 0air <1% 30 min Green 0.4713 44 4-10 969MSB 800° C. 1 N2 44.8  1 min 800°C. 0 air <1%  2 hour Green-Yellow 0.5226 122 4-11 969MSB 800° C. 1 N244.8 16 min 800° C. 0 air <1%  4 hour Yellow-Green 0.5694 257 4-12969MSB 800° C. 1 N2 44.8 15 min 800° C. 0 air <1%  8 hour Yellow-Green0.6119 496 4-13 969MSB 800° C. 1 N2 44.8 39 min 800° C. 0 air <1% 20hour Beige Orange 0.6634 1240 4-14 969MPI 560° C. 0 N2 ~30  5 min 788°C. 0 air ~20%   15 min Yellow-Tan 0.6542 20 4-15 969MPI 800° C. 0 CO ~5010 min 800° C. 0 air <1% 10 min Beige 0.5817 20 4-16 969MPI 800° C. 0 CO~50 10 min 800° C. 0 air <1% 10 min Beige 0.5736 20 4-17* 969MPI 600° C.0 Air ~40 15 min 800° C. 0 CO ~25   10 min Beige 0.6511 25 4-18 969MS300° C. 0 H2O N2 5 15 min 800° C. 0 air <1% 15 min Yellow-Tan 0.6201 304-19 969MS 400° C. 0 H2O N2 5 18 min 800° C. 0 air <1% 20 min Yellow-Tan0.6893 38 4-20 969MS 500° C. 0 H2O N2 5 16 min 800° C. 0 air <1% 18 minYellow-Tan 0.6978 33 4-21 969MS 600° C. 0 H2O N2 5 19 min 800° C. 0 air<1% 15 min Yellow-Tan 0.7185 34 4-22 969MS 700° C. 0 H2O N2 5 19 min800° C. 0 air <1% 15 min Yellow-Tan 0.6845 34 4-23 969MS 800° C. 0 H2ON2 5 19 min 800° C. 0 air <1% 18 min Yellow-Tan 0.6245 37

Control 1 shows the results of a typical 1-step lab activation, in whichthe temperature was ramped up to 800° C. at 60° C./hour, while thecatalyst was fluidized in dry air. The bed depth was only about 2.5inches (6.4 cm) in this test, which helped stabilization. The overallactivation time (cycle time) was rather long, but the stabilization washigh, due mainly to the small catalyst bed. Unfortunately, this exactsequence is not efficient enough for use on a large scale because of thelimited bed depth.

Control 2 shows the results obtained when the same catalyst was simplydropped into a hot 800° C. tube, while in dry air. The catalyst was thenallowed to fluidize at 800° C. for another 15 minutes before beingdischarged from the activator apparatus. This greatly reduced the cycletime, but this severe treatment also destroyed the catalyst, asindicated by the green color, the low Cr(+6) stabilization, and byactivity tests on the polymer (Table 2) which indicated complete loss ofactivity.

Control 3, shows a typical commercial activation in which the bed depthwas over 7 feet (2.1 meters). Although air velocity was higher in thisrun than the others, stabilization was still rather poor due to thelarge amount of catalyst used (750 lbs or 340 Kg). This much catalystreleased considerable moisture, which hurt the stabilization.Stabilization was low in spite of the exceedingly long cycle time of 36hours.

Samples 4-1 through 4-23 in Table 1 demonstrate other sequences usingactivation scenarios that are adaptable to continuous activation, or tolarge scale batch activation, while showing considerable increase inefficiency and in stabilization to Cr(+6). The parameters of the studiesare listed in Table 1 for the following examples.

Example 4A

For samples 4-1 through 4-7, the catalyst was fluidized in an oxidizingatmosphere of air and rapidly ramped up to a first temperature followedby a first hold period. The catalyst was then ramped up in an atmosphereof air to a second temperature that was greater than the firsttemperature and then held for a second hold period. Even at total cycletimes of less than 60 minutes the amount of Cr(6+) recovered was greaterthan 0.45 wt %.

Example 4B

For samples 4-8 through 4-14, catalyst was fluidized in an inertatmosphere of nitrogen and rapidly ramped up to a first temperaturefollowed by a first hold period. The catalyst was then ramped up in anatmosphere of air to a second temperature that was greater than or equalto the first temperature and then held for a second hold period. Even attotal cycle times of less than 20 minutes the amount of Cr(6+) recoveredwas commercially acceptable.

Example 4C

Samples 4-15 through 4-16, catalyst was fluidized in a reducingatmosphere of carbon monoxide and rapidly ramped up to a firsttemperature followed by a first hold period. The catalyst was thenramped up in an atmosphere of air to a second temperature that wasgreater than or equal to the first temperature and then held for asecond hold period. Even at total cycle times of about 20 minutes theamount of Cr(6+) recovered was commercially acceptable.

Example 4D

Sample 4-17, catalyst was fluidized in an oxidizing atmosphere ofambient air and rapidly ramped up to a first temperature followed by afirst hold period. The catalyst was then ramped up in an atmosphere ofair to a second temperature that was greater than or equal to the firsttemperature and then held for a second hold period. The catalyst wasthen subjected to a reducing atmosphere of CO for an additional 15minutes. Even at total cycle times of about 25 minutes the amount ofCr(6+) recovered was commercially acceptable.

Example 4E

Samples 4-18 to 4-23, catalyst was instantaneously added to a pre-heatedactivator in an oxidizing, reducing, or inert atmosphere and held for afirst period of time. The catalyst was then ramped up to a secondtemperature greater than the first temperature in an atmosphere of airand held for a second period of time. Stabilization to Cr(6+) wascommercially acceptable.

Polymerization reactions were conducted using the activated catalystrecovered from the examples and the resulting polymer was tested forvarious properties. Melt index (MI, g/10 min) was determined inaccordance with ASTM D1238 condition F at 190° C. with a 2,160 gramweight. High load melt index (HLMI, g/10 min) was determined inaccordance with ASTM D1238 condition E at 190° C. with a 21,600 gramweight.

Melt rheological characterizations were performed as follows.Small-strain (10%) oscillatory shear measurements were performed on aRheometrics Scientific, Inc. ARES rheometer using parallel-plategeometry. All rheological tests were performed at 190° C. The complexviscosity |η*| versus frequency (ω) data were then curve fitted usingthe modified three parameter Carreau-Yasuda (CY) empirical model toobtain the zero shear viscosity—η₀, characteristic viscous relaxationtime—τ_(η), and the breadth parameter—a. The simplified Carreau-Yasuda(CY) empirical model is as follows.

${{{\eta^{*}(\omega)}} = \frac{\eta_{0}}{\left\lbrack {1 + \left( {\tau_{\eta}\omega} \right)^{a}} \right\rbrack^{{({1 - n})}/a}}},$wherein:

-   -   |η*(ω)|=magnitude of complex shear viscosity;    -   η₀=zero shear viscosity    -   τ_(η)=viscous relaxation time    -   a=“breadth” parameter    -   n=fixes the final power law slope, fixed at 2/11; and    -   ω=angular frequency of oscillatory shearing deformation.        Details of the significance and interpretation of the CY model        and derived parameters may be found in: C. A. Hieber and H. H.        Chiang, Rheol. Acta, 28, 321 (1989); C. A. Hieber and H. H.        Chiang, Polym. Eng. Sci., 32, 931 (1992); and R. B. Bird, R. C.        Armstrong and O. Hasseger, Dynamics of Polymeric Liquids, Volume        1, Fluid Mechanics, 2nd Edition, John Wiley & Sons (1987); each        of which is incorporated herein by reference in its entirety. In        Table 2, the zero shear viscosity is reported as Eta 0, the        viscous relaxation time as Tau eta, and the breadth parameter as        “a”.

Table 2 demonstrates the results of polymerization runs with thesecatalysts. Melt index and HLMI are presented in grams/10 minutes. Theamount of polyethylene resin recovered is listed in grams under theheading “PEg”. Productivity is referred to as “Prod” in the table andwas determined by dividing the grams of resin produced by the grams ofcatalyst used for the polymerization run. Activity was determined bycalculating the productivity per hour (grams of resin/grams ofcatalyst/hour). The amount of catalyst charged to the reactor is labeledas “Cat Chg” and is in grams.

One can see that despite the faster rate of activation, the inventivecatalysts displayed equivalent or improved activity over the controlcatalysts, while providing higher melt index potential.

TABLE 2 Polymerization Runs Cat Run Prod Activity Melt Example # Chg PEg Time (g/g) (g/g-h) Index HLMI HLMI/MI Eta_0 Tau_eta “a” Control 10.1451 260 70 1793 1537 0.20 16.1 78.6 Control 2 0.0926 60 49 49 Control3 0.091 330 95 3626 2290 0.16 14.4 90.5 3.75E+05 1.23 0.1837 4-7 0.1937301 57 1554 1636 0.57 36.8 64.5 9.89E+04 0.19 0.1860 4-14 0.1119 224.250 2004 2404 0.25 18.7 74.0 4-16 0.1444 249 68.9 1724 1502 0.41 27.166.1 1.64E+05 0.31 0.1793 4-17 0.1417 262 60 1849 1849 0.14 12.9 89.73.72E+05 0.96 0.1834

The discussion of a reference herein is not an admission that it isprior art to the present invention, especially any reference that mayhave a publication date after the priority date of this application. Thedisclosures of all patents, patent applications, and publications citedherein are hereby incorporated by reference, to the extent that theyprovide exemplary, procedural or other details supplementary to thoseset forth herein. If the teachings or terms of the reference areinconsistent with those of the present invention, the informationincluded in the specification for the present invention is intended.

The inventive aspects described herein are exemplary only, and are notintended to be limiting. Many variations and modifications of theinvention disclosed herein are possible and are within the scope of theinvention. Use of the term “optionally” with respect to any element of aclaim is intended to mean that the subject element is required, oralternatively, is not required. Both alternatives are intended to bewithin the scope of the claim.

Accordingly, the scope of protection is not limited by the descriptionset out above but is only limited by the claims which follow, that scopeincluding all equivalents of the subject matter of the claims. Each andevery claim is incorporated into the specification as an embodiment ofthe present invention. Thus, the claims are a further description andare an addition to the preferred embodiments of the present invention.

1. A catalyst activation process comprising subjecting a chromiumcatalyst to a first stage comprising a first ramp up time to a firstdesired temperature of greater than 400° C., maintaining the firsttemperature for a first hold period, subjecting the catalyst to at leasta second stage comprising a second ramp up time to a second desiredtemperature, and maintaining the second temperature for a second holdperiod, wherein the second temperature is greater than the firsttemperature, and wherein the first stage is carried out in an inert orreducing atmosphere and the second stage is carried out in an oxidizingatmosphere, and further comprising following the second stage with athird stage comprising a third ramp up to a third temperature of lessthan 1000° C. and holding for a third hold period in an oxidizingatmosphere.
 2. The process of claim 1 wherein the inert atmosphere,reducing atmosphere, or oxidizing atmosphere are introducedindependently during the ramp up time, during the hold time, or duringboth the ramp up time and the hold time.
 3. The process of claim 1wherein the oxidizing atmosphere comprises oxidizing compounds, thereducing atmosphere comprises reducing compounds, and the inertatmosphere comprises inert compounds.
 4. The process of claim 3 whereinthe oxidizing compounds comprise ambient air, anhydrous air, oxygen,nitrous oxide, nitrogen dioxide, nitric oxide, oxygen containing halidecompounds, or any combination thereof; the reducing compounds comprisecarbon monoxide, hydrogen, materials that decompose to carbon monoxide,carbon, and hydrogen, or any combination thereof and the inertatmosphere comprises carbon dioxide, vacuum, helium, argon, nitrogen orany combination thereof.
 5. The process of claim 1 wherein theactivation process operates in a batch mode or a continuous mode.
 6. Theprocess of claim 1 wherein the time for any ramp up independentlycomprises a range from about 0 to about 3 hours.
 7. The process of claim1 wherein the first temperature is less than about 700° C. and thesecond temperature is greater than the first temperature and less thanabout 1000 ° C.
 8. The process of claim 1 wherein any hold timecomprises a range from about 1 minute to about 30 hours.
 9. The processof claim 1 wherein the time for any ramp up is instantaneous or about 0hours.
 10. The process of claim 1, wherein the first ramp up time isinstantaneous, the first temperature is less than 700° C., the firsthold period is from 1 minute to 30 hours, the second ramp up time isinstantaneous, the second temperature is greater than the firsttemperature and less than or equal to about 1000° C., and the secondhold period is from about 1 minute to about 30 hours.
 11. The process ofclaim 1 wherein the first stage is carried out in a first atmospherecomprising nitrogen, carbon monoxide, or combinations thereof, the firsttemperature is from about 400° C. to about 700° C., the first ramp uptime is from about 0 hours to about 1 hour, the first hold period isfrom about 1 hour to about 5 hours, the second stage is carried out in asecond atmosphere which is oxidizing, the second temperature is about750° C. to about 900° C., the second ramp up time is from about 0 hoursto about 1 hour, and the second hold period is about from about 1 hourto about 5 hours, wherein the second atmosphere is ambient airintroduced during the second hold period.
 12. The process of claim 1wherein the amount of water in the atmosphere during the hold period ofthe last performed stage of the process is less than about 50,000 ppm.13. The process of claim 1 further comprising adjusting the percentageof water in the atmosphere during the second stage of the activationprocess to less than about 30 mol %.
 14. The process of claim 1 furthercomprising subjecting the catalyst to a reducing atmosphere for a periodof time from about 10 minutes to about 5 hours following a lastperformed hold period of the activation process.
 15. The process ofclaim 1 wherein the process results in the formation of an activatedcatalyst and the activated catalyst comprises from 80% to 100%conversion of the total chromium to chromium (+6) after activation at750° C. to 900° C.
 16. The process of claim 1 having a total activationcycle of less than about 20 hours.
 17. The process of claim 1 whereinthe percentage of water in the atmosphere during the first stage is nomore than about 50 mol %, and wherein the percentage of water in theatmosphere during the second stage is no more than about 1 mol %. 18.The process of claim 1 wherein the percentage of water in the atmosphereduring the first stage is no less than about 5 mol %, and wherein thepercentage of water in the atmosphere during the second stage is no morethan about 1 mol %.
 19. The process of claim 1 wherein the chromiumcatalyst comprises at least 0.4 wt % chromium(+6) after the third stage,and wherein the sum of the first ramp up time, the first hold period,the second ramp up time, the second hold period, the third ramp up time,and the third hold period is no more than 400 minutes.
 20. The processof claim 1 wherein the chromium catalyst comprises at least 0.6 wt %chromium(+6) after the third stage, and wherein the sum of the firstramp up time, the first hold period, the second ramp up time, the secondhold period, the third ramp up time, and the third hold period is nomore than 8 hours.